Preparation of dialkyl oxalates using a heterogeneous Pd-V-P catalyst for the oxidative carbonylation of alcohols

ABSTRACT

Synthesis of dialkyl oxalates by the heterogeneous catalyzed oxidative carbonylation of liquid monohydric saturated alcohols of from 1 to 20 carbon atoms with carbon monoxide and oxygen or an oxygen-containing gas in the presence of a catalytic amount of a catalyst comprising palladium or a salt thereof in combination with a single phase crystalline vanadium-phosphorus containing compound consisting of vanadium in an oxide form and an oxide of phosphorus.

BACKGROUND OF THE INVENTION

The preparation of dialkyl oxalate esters by the homogeneous catalyzedoxidative carbonylation of alcohols in the presence of metal saltcatalysts, redox agents, dehydrating agents and other compounds such asamines, carbonates, nitrates, hydroxides and ureas is well known. Anarticle by Donald M. Fenton and Paul J. Steinwand, Journ. of Org. Chem.,Vol. 39, Nos. 5, 1974, pp. 701-704 describes a general mechanism for theoxidative carbonylation of alcohols to yield dialkyl oxalates using apalladium redox system, oxygen and dehydrating agents. Typical prior artpatents disclosing homogeneous catalyzed oxidative carbonylation ofalcohols to prepare oxalate esters are U.S. Pat. Nos. 3,393,136;3,994,960; 4,005,129; 4,005,130; 4,076,949; 4,118,589 and 4,281,174 aswell as West German Pat. No. 2,213,435 and West GermanOffenlegungschrift No. 2,601,139.

The present invention provides a much improved process for theproduction of dialkyl oxalates by employing an insoluble easilyrecoverable heterogeneous catalyst for the oxidative carbonylation ofalcohols.

U.S. Pat. No. 4,229,591 describes a process for the preparation of adiester of oxalic acid by contacting an ester of nitrous acid or analcohol and a nitrogen oxide or hydrate thereof in the gaseous phase inthe presence of a solid catalyst containing palladium or a salt thereofsuch as palladium on activated carbon.

Japanese Kokai No. 75-157,311 discloses the preparation of oxalic acidesters by reacting a C₁ to C₂₀ monohydric alcohol, carbon monoxide andmolecular oxygen in the presence of a supported Group VIII metal and aGroup IB, IIB, III, IV, V, VI, and VIIIB metal, aluminum, iron, cobaltor nickel.

U.S. Pat. No. 4,039,572 discloses the preparation of diesters ofdicarboxylic acids by the oxidative carbonylation of olefins andalcohols using a carrier supported catalyst consisting of (1) a platinumgroup metal compound and (2) a compound of a metal having an atomicnumber of not less than 22 which has been reduced to a metal and has aratio of (2) to (1) of from 0.0005:1 to 10:1 gram atoms.

The oxalate products of this invention have many commercial applicationsand are used as solvents, dye intermediates, for the preparation ofpharmaceuticals as well as feedstock for hydrogenation to ethyleneglycol by, for example, the process described in U.S. Pat. No.4,112,245.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a superiorimproved process for the synthesis of dialkyl oxalates by the liquid orvapor phase oxidative carbonylation of a liquid alcohol with a mixtureof carbon monoxide and oxygen in the presence of a catalytic amount ofan insoluble tri-metallic containing heterogeneous catalyst comprising acombination of (1) palladium metal or a salt thereof and (2) a singlephase crystalline vanadiumphosphorus compound consisting of vanadium inoxide form and an oxide of phosphorus. High yield and selectivity ofdialkyl oxalates is obtained especially with the lower alcohols. Inaddition, the Pd-V-P containing catalyst may be supported on, forexample silica (SiO₂) to provide still greater selectivity and catalystproductivity.

It is a primary object of this invention to provide a process for thepreparation of dialkyl oxalates in high yield and high conversion ofreactants employing an improved Pd-V-P containing heterogeneous catalystsystem.

It is a further object of this invention to provide a specificheterogeneous catalytic mechanism for the employment of palladium orsalts thereof with a single phase compound of vanadium in the oxide formand an oxide of phosphorus in an oxidative carbonylation processemploying alcohol, carbon monoxide and oxygen as reactants.

These and other objects and advantages of this invention will becomeapparent from the description of the invention which follows and fromthe claims.

DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that superior yields ofdialkyl oxalates at improved selectivities and catalyst productivity canbe obtained from the reaction of an alcohol and a mixture of carbonmonoxide, oxygen or an oxygen containing gas, such as air, at elevatedtemperatures and pressures in the presence of a solid catalytic mixtureof palladium or a salt thereof, in combination with a crystallinecompound consisting of vanadium in the form of its oxide and an oxide ofphosphorus. The Pd-V-P containing catalyst is in the heterogeneous statein the reaction at reaction conditions and while vapor phase reactionsemploying a bed of catalyst may be employed the reactions are generallycarried out with a slurry of the catalyst mixture, supported orunsupported, in the reactant alcohol. The catalyst may be on inertsupport materials such as alumina, silica gel, aluminosilicates,activated carbon or zeolites, and when so employed will generallyprovide a higher percent selectivity of product and greater catalystproductivity.

The reaction between the alcohol, carbon monoxide, and oxygen may becarried out in an autoclave or any other high pressure reactor. Althoughthe order of addition of reactants and the Pd-V-P containing catalystmay vary, a general procedure is to charge the alcohol and catalyst(supported or unsupported) into the reaction vessel, and then introducethe proper amount of carbon monoxide and oxygen to the desired reactionpressure and then heat the mixture to the desired temperature for theappropriate period. The reaction can be carried out batchwise or as acontinuous process and the order of addition of the reactants may alsobe varied to suit the particular apparatus employed. The addition of theoxygen or oxygen-containing gas, such as air, can be a pulsed orcontinuous addition to the reaction system. The reaction products arerecovered and treated by any conventional method such as distillationand/or filtration, etc. to effect separation of the oxalate ester fromunreacted materials, catalyst, by-products, etc.

The alcohols which may be employed in concentrations of from about 50 to99.7 weight percent, preferably 77 to 94 weight percent and suitable foruse in the process of the present invention can be monohydric saturatedaliphatic and alicyclic alcohols and may contain other substitutentssuch as amido, alkoxy, amino, carboxy, cyano, etc. radicals in additionto the hydroxyl group. The substituents, in general, do not interferewith the reaction.

The alcohols which are employed may be primary, secondary, or tertiaryalcohols and conform to the general formula ROH, wherein R is anoptionally substituted aliphatic, or alicyclic group containing from 1to 20 carbon atoms and preferably unsubstituted aliphatic alcoholscontaining from 1 to 10 carbon atoms and more preferably 1 to 4 carbonatoms. In general, the alcohol is one which is normally liquid under theconditions employed in the carboxylation reaction. Representativealcohols especially suitable for use in this invention are saturatedmonohydric alcohols such as methyl, ethyl, n-, iso-, sec-, andtert-butyl, amyl, hexyl, octyl, lauryl, n- and iso-propyl, cetyl,benzyl, chlorobenzyl and methoxy-benzyl alcohols as well as for example,cyclohexanol, heptanols, decanols, undecanols, 2-ethyl hexanol, nonanol,myristyl alcohol, stearyl alcohol, methyl cyclohexnol, pentadecanol,oleyl and eicosonyl alcohols, and the like. The preferred alcohols arethe primary and secondary monohydric saturated aliphatic alcohols, suchas methanol, ethanol, 1- and 2-propanol, n-butyl alcohol etc., up to 10carbon atoms.

The palladium salts which may be employed in the process of thisinvention and in forming the catalyst mixture include the palladium (II)compounds or mixtures thereof. Among the chemical forms of the palladiumcompounds which can be used as such or as mixtures are the palladium,halides, sulfates, carboxylates, acetates, oxides, and nitrates,preferably the palladium (II) halides. Representative palladium saltcompounds include, for example palladium (II) oxide, palladium (II)chloride, palladium (II) sulfate, palladium (II) acetate, palladium (II)iodide, palladium (II) oxalate, palladium (II) propionate, etc. Thepalladium content of the catalyst system may range from about 0.25 to 5weight percent on the crystalline vanadium oxide-phosphorus oxidecompound.

The vanadium employed in the preparation of the crystalline catalystmixture is in the essentially insoluble oxide form and may be V₂ O₅, V₂O₄ or V₂ O₃ or mixture thereof as well as pentavalent salts such asammonium metavanadate.

The oxides of phosphorus employed in the process of this invention andin forming the catalyst mixture include phosphoric acid, phosphorustrioxide (or phosphorus anhydride) P₄ O₆, phosphorus pentoxide (orphosphoric anhydride) P₂ O₅ as well as oxides of phosphorus which may bein the form of P₄ O₇, P₄ O₈ or P₄ O₉ or combinations thereof, all ofwhich are generally produced by the direct oxidation of phosphorus. Aparticularly useful combination of single phase crystallinevanadium-phosphorus which may be employed in the catalyst system issingle phase crystalline vanadium(IV)bis(metaphosphate), VO(PO₃)₂ whichmay be prepared, for example, by the methods described in U.S. Pat. Nos.4,165,299 and 4,247,419 and incorporated herein by reference or themixed V-P oxide catalysts of U.S. Pat. No. 4,333,853. The vanadiumphosphorus ratio in the crystalline compound will range from about 1:0.5to about 1:3 and is preferably a ratio of 1:2.

While methods for the preparation of the Pd-V-P supported or unsupportedcatalyst is set forth hereinafter in the examples, other known methodsfor the preparation of such catalyst mixture may be used as long as theyproduce a Pd-V-P catalyst having an appropriate ratio of one metal tothe other in the catalyst system. Generally, the atomic ratio of the Pdto V to P as metals in the catalyst employed will range from about0.01:1:0.5 to about 0.2:1:3 and is preferably 0.05:1:2. The reaction isgenerally carried out in the presence of a catalytic proportion of theheterogeneous catalyst combination and will proceed with small amountsof the catalyst hereinabove described. Generally the proportions ofcatalyst used in the reaction as a slurry mixture with alcohol will beequivalent to between about 0.05 to 5 weight percent based on thealcohol employed and is preferably employed in amounts of between 0.5 to2.5 percent by weight of the alcohol employed.

Although not required, solvents, if desired, which are chemically inertto the components of the reaction system may be employed. Suitablesolvents include, for example, hydrocarbons such as hexane, heptane,toluene and xylene; ethers such as tetrahydrofuran, diethylether,diphenylether; halogenated hydrocarbons such as methylene chloride,chlorobenzene and dichlorobenzene; organic esters such as ethyl acetate,n-propyl formate, isopropyl acetate, sec- and iso-butyl acetate, amylacetate, cyclohexyl acetate, n-propyl benzoate; lower alkyl phthalates,etc. and the alkyl sulfones and sulfoxides such as propylene ethylsulfoxide, diisopropyl sulfone, diisooctyl sulfoxide, acetone,cyclohexanone, etc.

As indicated above the reaction can be suitably performed by introducingthe oxygen and carbon monoxide at a desired pressure into contact withthe alcohol/catalyst mixture comprising the palladium-vanadiumoxide-phosphorus oxide catalyst either supported or unsupported andheating to the desired temperature. In general, a carbon monoxidepressure of about 400 psig to about 5000 psig partial pressure andpreferably from 900 psig to about 2200 psig is employed. Stoichiometricquantities of carbon monoxide are generally employed. However, an excessof carbon monoxide may be employed, for example, in continuous processeswhere large excess of or high carbon monoxide requirements are generallyutilized, a suitable recycle of the unreacted carbon monoxide may beemployed. The reaction will proceed at temperatures of from about 40° C.to 150° C. It is generally preferred to operate the process attemperatures in the range of 75° C. to 120° C. to obtain a convenientrate of reaction. Heating and/or cooling means may be employed interiorand/or exterior of the reaction to maintain the temperature within thedesired range.

At least stoichiometric amounts of oxygen or an oxygen-containing gassuch as air may be employed and at any oxygen partial pressure such thatthe explosive range is avoided. Thus, the concentrations of oxygenshould be low enough so that the reaction mixture is not potentiallyexplosive. The Handbook of Chemistry and Physics, 48th Edition, 1967indicates that the explosive limits of pure oxygen in carbon monoxide is6.1 to 84.5 volume percent and air in carbon monoxide to be 25.8 to 87.5volume percent. The volume percent of the oxygen in the oxygen-carbonmonoxide mixture usually amounts to about 3 to 6 percent. In carryingout the reaction the oxygen is charged to the reaction vessel to thedesired pressure and concentration and may be charged in portions forsafety reasons. Total carbon monoxide-oxygen pressures will rangebetween about 500 psig and 6000 psig.

The reaction time is generally dependent upon the alcohol being reacted,temperature, pressure and on the amount of the catalyst mixture beingcharged as well as the type of equipment being employed. Reaction timeswill vary dependent on whether the process is continuous or batch butwill generally run for from 0.5 to 2 hours under batch conditions. Thereaction is limited by the available oxygen, alcohol and carbonmonoxide.

The following examples are provided to illustrate the invention inaccordance with the principles of this invention but are not to beconstrued as limiting the invention in any way except as indicated bythe appended claims.

EXAMPLE 1 Preparation of a Pd-V-P Containing Catalyst

A 1 liter, three necked flask equipped with a mechanical stirrer, acondenser with a Dean-Stark water trap, a thermometer and an additionfunnel was charged with 60 g. of vanadium pentoxide, 240 ml of isobutylalcohol and 160 ml of benzyl alcohol. The resulting heterogeneousmixture was stirred at reflux for 4 hours during which time 2 ml ofwater was collected in the trap. The suspended solids underwent a colorchange from brown to black indicative of a V^(V) to V^(IV) reduction.The slurry was then cooled to room temperature and a solution of 91.28g. of 85 percent ortho-phosphoric acid in 80 ml of isobutanol addedslowly over a period of 45 minutes via the addition funnel. A slightexotherm occurred. The resulting mixture was stirred at reflux for 8hours over which time 15 ml of water was removed. The suspended solidsunderwent a color change of from black to blue 30 minutes aftercompletion of the phosphoric acid addition. After cooling to roomtemperature, the solids were collected on a Buchner funnel, washed with100 ml of isobutanol and air dried for 3 hours. Residual solvents wereremoved by heating in a vacuum oven at 100° C. for 5 hours. Theresulting hard, green solids were ground to a powder and calcined from100° C. to 400° C. at a rate of 1° C. per minute and then held at 400°C. for 16 hours. The weight of the calcined green crystalline solidsequaled 110 g. 19.40 g. of the crystalline vanadium-phosphorus solidmaterial was added to a 35 ml solution of 1N ammonium hydroxidecontaining 0.999 g. of palladium chloride dissolved therein. Theresulting slurry was stirred for 1 hour at room temperature and thenheated on a steam bath with stirring to near dryness. After drying at120° C. for 2 hours, the catalyst was calcined from 100° C. to 400° C.at a rate of 1° C. per minute and then held at 400° C. for 16 hours. Theresulting catalyst weight equaled 19.90 g. The atomic ratio of palladiumto vanadium to phosphorus as metal in the catalyst was 0.05:1:2.

A SiO₂ supported Pd-V-P containing catalyst was prepared in accordancewith the above procedure by adding 110 g. of silica gel to the vanadiumpentoxideisobutanol-benzyl alcohol mixture, prior to reflux and additionof the orthophosphoric acid, to give a catalyst containing a palladiumto vanadium to phosphorus atomic ratio of 0.066:1:1.2 on silica.

EXAMPLE 2 (Comparative)

To a 500 cc stainless steel stirred autoclave was charged 200 g. ofmethanol and 2.5 g. of a palladium-vanadium oxide catalyst (1 percent Pdand 56 percent V as the metal). The autoclave was brought to atemperature of 100° C. and 1200 psig of carbon monoxide added withstirring. 400 psig air was charged and then 900 psig carbon monoxide tobring the total pressure to 2500 psig. The reaction was carried out for1 hour after which the reactor was cooled to ambient temperature andvented to ambient pressure and gas samples obtained. Solids wereseparated from liquid products by vacuum filtration. The liquid productwas analyzed by gas-liquid chromatography (glc) and titration methodsand the gaseous product was analyzed by gas chromatograph. Analysis ofthe products showed 4.86 mmole dimethyl oxalate, 12.6 mmole methylformate and 0.6 mmole carbon dioxide. Selectivity to dimethyl oxalatewas 42.4 percent with a catalyst productivity of 0.573 g/g-hr.

EXAMPLE 3 (Comparative)

The procedure of Example 2 was repeated using 2.5 g. of 2 weight percentpalladium metal on silica as catalyst. The reaction was carried out for1 hour under the same temperature and pressure conditions of Example 2.Analysis of the reaction products showed a selectivity to dimethyloxalate of 35.7 mole percent with a catalyst productivity of 0.18g/g-hr.

EXAMPLE 4

To a 500 cc stainless steel stirred autoclave equipped with an internalcooling coil was added a slurry of 200 g. of methanol and 2.5 g. ofsilica supported Pd-V-P catalyst as prepared by Example 1. While heatingthe autoclave to 100° C., 1200 psig carbon monoxide followed by 400 psigair and another 900 psig carbon monoxide was added with stirring tobring the total pressure to 2500 psig. The reaction was carried out for15 minutes after which the reactor was cooled by pumping isopranolcooled in dry ice through the internal coils. A temperature of -10° C.was obtained in 30 minutes. The autoclave was vented to ambient pressureand gas samples obtained. The liquid product was separated from solidsby vacuum filtration. The liquid product was analyzed by gas-liquidchromatography (glc) and titration methods and the gaseous product wasanalyzed by gas chromatograph. Analysis of the products showed 14.2mmole dimethyl oxalate, 1.2 mmole oxalic acid and 0.6 mmole carbondioxide. Selectivity to dimethyl oxalate was 95 mole percent with acatalyst productivity of 2.41 g/g-hr.

EXAMPLE 5

The procedure and operating conditions of Example 4 was repeated exceptthat 2.5 g. of unsupported Pd-V-P catalyst as prepared by Example 1 andhaving a Pd:V:P atomic ratio of 0.05:1:2 was employed. Analysis of thereaction products showed 10.8 mmole dimethyl oxalate, 0.9 mmole oxalicacid and 0.4 mmole carbon dioxide. Selectivity to dimethyl oxalate was94 mole percent with a catalyst productivity of 2.21 g/g-hr.

EXAMPLES 6 TO 45

In Examples 6 to 45 which follow in table form Table 1, 2, 3 & 4), theprocedure of Example 4 was repeated using, 2.5 g. of an unsupported 3weight percent palladium containing crystalline vanadium-phosphoruscatalyst with various V:P atomic ratios, 200 gms. of various alcoholreactants and varying conditions with a 15 minute reaction time.Products were analyzed by gas liquid chromatography and titrationmethods to give mole percent selectivity to the dialkyl oxalate andcatalyst productivity in gram/gram-hour.

                                      TABLE 1                                     __________________________________________________________________________    Example No.                                                                              6    7   8   9   10  11  12  13  14  15                            __________________________________________________________________________    V:P Atomic Ratio                                                                         1:2  1:2 1:1.2                                                                             1:1.2                                                                             1:0.5                                                                             1:0.5                                                                             1:2 1:2 1:1.2                                                                             1:1.2                         Alcohol    MeOH.sup.(1)                                                                       MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                                                                              MeOH                          Pressure (psig)                                                               Air        240  480 320 400 320 400 400 400 400 400                           CO         1260 2520                                                                              1680                                                                              2100                                                                              1680                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                          Temp. (°C.)                                                                       100  100 100 100 100 100 75  125 75  125                           Products (mmoles)                                                             Dialkyl Oxalate                                                                          6.8  13.9                                                                              4.1 5.4 12.5                                                                              13.1                                                                              4.0 10.2                                                                              2.0 5.1                           Oxalic Acid                                                                              0.7  2.0 0.4 0.4 1.0 4.5 0.4 1.0 0.2 0.5                           CO.sub.2   0.3  0.5 0.8 0.8 0.7 0.7 0.4 0.5 0.8 1.0                           Selectivity (Mole %)                                                                     93   88  83  87  91  92  84  73  71  66                            Catalyst Productivity                                                                    1.41 3.0 0.85                                                                              1.10                                                                              2.56                                                                              3.33                                                                              0.84                                                                              2.12                                                                              0.42                                                                              1.06                          (g/g-hr.)                                                                     __________________________________________________________________________     .sup.(1) MeOH = Methyl Alcohol                                           

                                      TABLE 2                                     __________________________________________________________________________    Example No.                                                                              16  17  18   19   20   21  22  23  24  25                          __________________________________________________________________________    V:P Atomic Ratio                                                                         1:0.5                                                                             1:0.5                                                                             1:2  1:2  1:2  1:1.2                                                                             1:1.2                                                                             1:1.2                                                                             1:0.5                                                                             1:0.5                       Alcohol    MeOH                                                                              MeOH                                                                              EtOH.sup.(2)                                                                       PrOH.sup.(3)                                                                       BuOH.sup.(4)                                                                       EtOH                                                                              PrOH                                                                              BuOH                                                                              EtOH                                                                              PrOH                        Pressure (psig)                                                               Air        400 400 400  400  400  400 400 400 400 400                         CO         2100                                                                              2100                                                                              2100 2100 2100 2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                        Temp. (°C.)                                                                        75 125 100  100  100  100 100 100 100 100                         Products (mmoles)                                                             Dialkyl Oxalate                                                                          6.1 15.9                                                                              7.8  4.0  0.9  3.9 2.0 0.5 12.0                                                                              6.0                         Oxalic Acid                                                                              0.6 1.0 0.8  0.4  0.1  0.4 0.2 0.0 1.0 0.6                         CO.sub.2   0.7 1.2 0.4  0.5  0.6  0.8 1.0 1.5 0.8 0.7                         Selectivity (Mole %)                                                                     82  77  90   75   28   81  59  17  85  70                          Catalyst Productivity                                                                    1.27                                                                              3.20                                                                              2.01 1.22 0.33 1.00                                                                              0.61                                                                              0.17                                                                              3.03                                                                              1.83                        (g/g-hr.)                                                                     __________________________________________________________________________     .sup.(2) EtOH = Ethyl Alcohol                                                 .sup. (3) PrOH = nPropyl Alcohol                                              .sup.(4) BuOH = nButyl Alcohol                                           

                                      TABLE 3                                     __________________________________________________________________________    Example No.                                                                              26  27  28  29  30  31  32  33  34  35                             __________________________________________________________________________    V:P Atomic Ratio                                                                         1:0.5                                                                             1:2 1:2 1:1.2                                                                             1:1.2                                                                             1:0.5                                                                             1:0.5                                                                             1:2 1:2 1:1.2                          Alcohol    BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              EtOH                                                                              EtOH                                                                              EtOH                           Pressure (psig)                                                               Air        400 400 400 400 400 400 400 400 400 400                            CO         2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                                                                              2100                           Temp. (°C.)                                                                       100  75 125 75  125 75  125 75  125 75                             Products (mmoles)                                                             Dialkyl Oxalate                                                                          1.4 1.3 2.2 0.7 1.2 2.0 3.4 2.9 2.3 1.5                            Oxalic Acid                                                                              0.1 0.1 0.2 0.0 0.1 0.2 0.3 0.3 0.2 0.1                            CO.sub.2   1.2 0.2 0.8 0.5 2.0 0.4 1.6 0.1 0.9 0.2                            Selectivity (Mole %)                                                                     26  70  41  50  28  71  40  97  69  89                             Catalyst Productivity                                                                    0.50                                                                              0.46                                                                              0.79                                                                              0.24                                                                              0.41                                                                              0.70                                                                              1.20                                                                              0.74                                                                              0.60                                                                              0.37                           (g/g-hr.)                                                                     __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Example No.                                                                              36  37  38  39  40  41  42  43  44  45                             __________________________________________________________________________    V:P Atomic Ratio                                                                         1:1.2                                                                             1:0.5                                                                             1:0.5                                                                             1:2 1:2 1:1.2                                                                             1:1.2                                                                             1:0.5                                                                             1:0.5                                                                             1:1.2                          Alcohol    EtOH                                                                              EtOH                                                                              EtOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              BuOH                                                                              MeOH                           Pressure (psig)                                                               Air        400 400 400 320 480 320 480 320 480 240                            CO         2100                                                                              2100                                                                              2100                                                                              1680                                                                              2520                                                                              1680                                                                              2520                                                                              1680                                                                              2520                                                                              1260                           Temp. (°C.)                                                                       125  75 125 100 100 100 100 100 100 100                            Products (mmoles)                                                             Dialkyl Oxalate                                                                          1.2 4.4 3.4 1.1 2.7 0.6 1.5 1.6 4.2 3.4                            Oxalic Acid                                                                              0.1 0.4 0.4 0.1 0.3 0.0 0.1 0.2 0.4 0.3                            CO.sub.2   1.7 0.2 1.7 0.7 0.7 1.8 1.8 1.4 1.4 0.7                            Selectivity (Mole %)                                                                     37  96  64  37  58  19  38  34  55  84                             Catalyst Productivity                                                                    0.30                                                                              1.12                                                                              0.90                                                                              0.40                                                                              0.99                                                                              0.20                                                                              0.51                                                                              0.60                                                                              1.50                                                                              0.70                           (g/g-hr.)                                                                     __________________________________________________________________________

We claim:
 1. A process for the preparation of dialkyl oxalates by theoxidative carbonylation of a liquid saturated monohydric aliphatic oralicyclic alcohol containing from 1 to 20 carbon atoms with a mixture ofcarbon monoxide and oxygen or an oxygen containing gas at a temperatureof from about 40° C. to 150° C. and a total pressure of between about500 psig and 6000 psig in the presence of a catalytic amount of anessentially insoluble heterogeneous tri-metallic-containing catalystcomprising palladium or a salt thereof in combination with a singlephase crystalline vanadium-phosphorus containing compound consisting ofa vanadium oxide and an oxide of phosphorus said catalyst containing asthe metals an atomic ratio of palladium to vanadium to phosphorus offrom about 0.01:1:0.5 to about 0.2:1:3 and recovering the desireddialkyl oxalate.
 2. A process according to claim 1 wherein the alcoholis a monohydric aliphatic alcohol containing from 1 to 10 carbon atoms.3. A process according to claim 2 wherein the alcohol is selected fromthe group consisting of methanol, ethanol, propanol and butanol.
 4. Aprocess according to claim 3 wherein the alcohol is methanol.
 5. Aprocess according to claim 1 wherein the ratio is 0.05:1:2.
 6. A processaccording to claim 1 wherein the vanadium-phosphorus containingcrystalline compound has an atomic ratio of V:P of from about 1:0.5 toabout 1:3.
 7. A process according to claim 6 wherein the ratio is 1:2.8. A process according to claim 7 wherein the vanadium-phosphoruscontaining crystalline compound is vanadium(IV)bis(metaphosphate).
 9. Aprocess according to claim 1 wherein the Pd-V-P catalyst is employed inamounts of from about 0.05 to 5 weight percent based on the alcoholemployed.
 10. A process according to claim 9 wherein between 0.5 to 2.5weight percent catalyst is employed.
 11. A process according to claim 1wherein the temperature is between about 75° C. and 120° C. and carbonmonoxide partial pressure is between 900 psig and 2200 psig.
 12. Aprocess according to claim 1 wherein the reaction is carried out in asolvent inert to the components of the reaction system.
 13. A processaccording to claim 1 wherein the Pd-V-P containing catalyst issupported.
 14. A process according to claim 13 wherein the support isSiO₂.
 15. A process for the preparation of dimethyl oxalate by theoxidative carbonylation of methanol with carbon monoxide and oxygen oran oxygen-containing gas at a temperature of from 75° C. to 120° C. anda carbon monoxide pressure of 2100 psig and oxygen or oxygen-containinggas pressure of 400 psig in the presence of a catalytic amount of atri-metallic-containing catalyst comprising palladium or a salt thereofcombined with a single phase crystalline vanadium-phosphorus compoundconsisting of a vanadium oxide and an oxide of phosphorus said catalystbeing present in the heterogeneous phase and containing as the metals anatomic ratio of palladium to vanadium to phosphorous of from about0.01:1:0.5 to about 0.2:1:3.